In wireless communication systems, adaptive antenna arrays are used for offering significant capacity improvements, especially in an interference-limited environment. See, e.g., J. Liberti, T. Rappaport, “Analytical results for capacity improvements in CDMA”, IEEE Transactions on Vehicular Technology, vol. 43, pp. 680-690, 1994, and J. Winters et al., “The impact of antenna diversity on the capacity of wireless communication systems,” IEEE Transactions on Communications, vol. 42, pp. 1740-1751, 1994. This technology offers the ability to eliminate same cell interference for mobile stations being served simultaneously. It offers the prospect of a reduction of inter-cell interference. It also increases the signal-to-noise ratio of a particular mobile station being served and therefore enables an increase in user data rate. These benefits and advantages result in either higher data throughputs, or the ability to service more mobile stations simultaneously, within a given cell or service infrastructure.
Adaptive antenna array is used to improve the performances of wireless communication systems. There are two types of adaptive antenna array: diversity antenna array and beamforming antenna array. In a diversity antenna array, the data stream are coded in space and time and sent from multiple low-correlated antennas to achieve diversity gain. On the other hand, beamforming array utilizes the spatial directivity and provide beamforming gain. Spatial directivity requires a good correlation among antennas.
The multiple antennas of the array are typically deployed at the base station of each cell, and the signals transmitted or received by the antennas are linearly combined with certain complex weights. Different antenna weights are used to extract the signals transmitted to or received from different mobile stations within the cell. By properly adjusting the antenna weights, the multiple antennas can improve the signal-to-interference ratio (SIR) through beamforming, interference cancellation and receive diversity.
With spatially separated antennas in the antenna array, beamforming becomes practical for both transmit and receive modes. Focusing radiant energy in the direction of a mobile station reduces the amount of overall power needed to be generated by the base station. Antenna array technology can be used to focus power coming from the mobile station to the base station via a reverse link or an uplink, as well as from the base station to the mobile station via a forward link or downlink.
Usually, during transmit mode, a wide transmit beam is desired so that the transmit beam, and its associated pilot, reaches all of the mobiles within the service area or sector, since the base station does not initially know where any particular mobile would be within that area. After a particular mobile station is located within the service area of the base station, narrower transmit beams may be employed to divide and concentrate limited base station power among all of the mobile stations being served simultaneously.
In base station receive mode, very narrow beams are highly desirable in order to provide multiple beam diversities and concentrate the signal energy from a particular one of the mobiles operating within a particular one of the available service channels and to exclude or reduce signal energies from other mobiles within the same service area using other ones of the available service channels. Narrow beamforming creating very narrow beams with high antenna array gains at the base station for both receive and transmit modes typically requires more antenna elements.
In the conventional art, two antenna arrays may be used with each one performing beamforming separately. Then, taking each antenna array as a single antenna independent from the other, a transmit diversity process is performed between these two antenna arrays. An alternative to that is to use one antenna array, but calculate two weights from the uplink signal for the antenna array. These two weights are then applied to the transmit signal to form two transmitted signals, forming two ‘virtual antennas’. Transmit diversity will then be implemented for these two ‘virtual antennas’.
Transmit beam forming requires a good correlation among antennas to achieve beam forming gain, while transmit diversity scheme requires independent or low correlation among antennas to achieve diversity gain. Apparently, these two requirements, that is, good antenna correlation for beam forming and low antenna correlation for diversity, are contradictive.
Furthermore, the antenna correlation has a dynamic characteristic as it depends on the air channel between the antenna array and the antenna on the mobile terminal that it communicates with. For example, in a case of point-to-multipoint systems (PMP, like cellular), for terminals at locations with line-of-sight or near line-of-sight condition, the base station antennas will have high correlations. On the contrary, if the terminals are located with non-line-of-sight conditions and the channels experience heavy multi-paths, the base station antennas will have no or little correlation.
Conventional methods do not take into account the antenna correlations when combining beamforming and transmit diversity for an antenna array. For example, some conventional methods use two transmitter antenna arrays (or one transmitter antenna array with a fixed sub-array partition). The fixed sub-array partition does not take into account the antenna correlations, and does not change or update over time. Some other conventional methods do not physically partition the antenna array into two sub-arrays. Instead, they use two weights (i.e., beamforming) to form two virtual antennas, and apply diversity between these two virtual antennas. Weights are calculated through covariance matrices. They need beamforming for all antennas and the antenna correlations are not taken into account.
Therefore, there exists a need to provide an improved wireless communications system benefiting simultaneously from both diversity and beamforming gains, taking into account the correlations among antennas.